Groundwater-surface water interactions can create microhabitats that affect biological productivity in streams, wetland-stream systems, lakes, and the coastal ocean. Groundwater discharge to surface water can also be a significant source of solute mass that affects chemical mass budgets and local ecology. Unfortunately, it is typically quite difficult to characterize water flux across the sediment-aquifer boundary because of spatial heterogeneity and temporal variability. Temporal variability in groundwater-surface water flux can be quite substantial, occurring over time scales of hours, days, weeks, seasons, and years.
Generally, there are three approaches to quantifying flow between aquifers and surface water bodies: (1) direct measurement; (2) hydrogeologic modeling; and (3) indirect measurement. Direct measurement techniques include both manual and automated seep meters. Manual meters are very labor intensive but they sample the flow directly through observing volume changes in an attached pre-filled bag. Because of low seepage rates and high labor requirements, sampling with manual seepage meters generally occurs over a few days with each sample representing an integrated flow over several hours. Therefore, manual seep meters cannot resolve either very short or long term temporal seepage patterns. Furthermore, factors associated with seep meter design (e.g., head loss through the collection chamber) and the external environment (e.g., waves and currents) can affect results and lead to large uncertainties. Automated seep meters may overcome many of the problems associated with manual meters, are less labor-intensive, and allow for better temporal resolution of seepage. However, they tend to be expensive, have high power requirements, and are subject to biofouling and therefore need frequent maintenance while deployed. Consequently, they too are not suited for measuring long-term seepage patterns.
Hydrogeologic modeling of groundwater-surface water interactions can take several different forms depending on the available data and scale of interest: Darcy's law, water budgets, hydrograph separation, and numerical modeling have all been used to calculate water fluxes. Darcy's law calculations can be used locally to determine the flux between two locations or can be used more regionally to determine large-scale flow through a system. Similarly, water budgets and hydrograph separation techniques lead to flow estimates over larger scales. Regional estimates of flow do not provide details within small spatial scales, such as within the hyporheic zone or shallow tidally-induced flow, where many important biogeochemical processes occur. Numerical models can be used for both large and small scale studies and can accommodate temporal variability. While such models offer a wider ability to characterize surface water—groundwater interactions, they can only be as good as the data used to build them and they require significant field data. Hence, numerical modeling, while potentially powerful, can be limited by data needs.
Indirect flux estimation techniques measure independent variables from which flux is calculated and typically include some type of natural tracer. The mass balance tracer methods result in spatially integrated flux estimates and therefore do not provide spatial variability information. In addition, spatial and temporal variability in groundwater tracer concentrations can lead to significant uncertainties in the resulting flux estimates. Inverse modeling using one or more tracers can provide point estimates that can be difficult to use for large-scale flow estimation when spatial heterogeneity is high. Furthermore, there is uncertainty in the modeling because many hydrogeologic parameters, such as permeability, dispersivity, porosity, and anisotropy, are all unknown yet must be included in the model. Accordingly, there is a need for improved systems and methods for measuring and quantifying flow between aquifer and surface water bodies.